An image projection apparatus uses at least three light emitting elements emitting primary colors (conventionally: red, green and blue, but not limited thereto) for displaying an image. The image may be a still image or a moving image (video) constructed of a sequence of (still) images. In order to create a video of sufficient quality for the human eye, a sequence frequency must be sufficiently high, where conventionally an image sequence rate for moving pictures of 24 Hz (film), 25 Hz (film on PAL standard, and some video), 30 Hz (film converted to NTSC standard), 50 Hz (video in PAL, often interlaced), 60 Hz (video in NTSC standard, often interlaced, frequently used in computer graphics) is used depending on the adopted standard in the relevant market. Higher frequencies are also used by some picture processing in a display device or on a computer to improve the quality of the video by enhancing the performance in moving images.
As is known from the prior art, e.g. from US 2006/0203204, according to the sequence frequency, within a time frame for constructing one image (image frame period), sequentially a red light emitting element, a green light emitting element, and a blue light emitting element are driven to illuminate a (achromatic) display panel which modulates the light for each pixel of an image to be constructed. From this publication it is further known that the light output (brightness) from a light emitting element, such as a light emitting diode (LED), may vary as a function of the temperature thereof. As a temperature of the light emitting element increases, its light output decreases. The degree of reduction of the light output depends on the type of the light emitting element, and its specific structure. It is known that in particular a red light emitting element suffers from a high temperature sensitivity, and may be the most critical color with regard to a drop of light output with increasing temperature. Green and blue light emitting elements have lower temperature sensitivities.
If no specific measures are taken, the temperature sensitivity of the light emitting elements causes the colors of an image to change over time, when the light emitting elements heat up: the light output (brightness) decreases differently for light emitting elements of different colors, and as a result a color formed by the addition of the colors generated by the different light emitting elements changes over time. This is undesirable.
According to US 2006/0203204, such problem may be solved by varying the pulse amplitude and/or the pulse width of the pulses driving the respective light emitting elements depending on the temperature of the light emitting elements such that a white balance of the generated image is retained. However, this requires a feedback control of the light emitting element driving means, and the storage of data regarding a temperature dependency of the light output of the light emitting elements. Additionally, since the maximum light output of a display device is limited by the maximum brightness of its weakest source, the control has to reduce the other colors in brightness, and overall performance is reduced.